Electric arc welding is often performed with a continuous welding wire including an outer metal sheath of low carbon steel and a core formed of flux material. The flux is not magnetic and is used to form a barrier over the weld bead to prevent atmospheric contamination during the welding and an inferior weld bead. To provide uniform welding, the percentage of core material, or “fill”, to metal in the sheath is held constant along the total length of the welding wire. However, as shown in Landis U.S. Pat. No. 3,466,907 the sheath is wrapped around the fill and then the cored electrode is drawn through sizing dies to manufacture a welding wire having a specific outer diameter. It has been found that the core material is not uniformly distributed into the arcuate shaped sheath prior to its being wrapped into a tube. Consequently, as the tube is drawn through a die having a fixed diameter the internal volume of core material controls the thickness of the outer sheath. A large volume of non-magnetic core material results in a relatively thin sheath, whereas a low volume of fill produces a relatively thick sheath during the drawing process that reduces the diameter of the sheath to the desired wire diameter. Thus, the thickness of the sheath varies with the volume of fill. The percentage of fill to metal is determined by the volume of core material. To effect a uniform, high quality weld, the percentage of fill must be held relatively constant. Heretofore, there has been no apparatus or procedure to indicate unacceptable variations in the volume of internal core material. This is a disadvantage experienced in manufacturing cored welding wire, since the fill volume is inversely proportional to the wall thickness.